add love.exposed and love.occluded event callbacks.

Add love.window.isOccluded.

Resolves #2004.

src/modules/window/sdl/Window.h

@@ -98,6 +98,7 @@ public:
 	bool hasMouseFocus() const override;
 
 	bool isVisible() const override;
+	bool isOccluded() const override;
 
 	void setMouseGrab(bool grab) override;
 	bool isMouseGrabbed() const override;